BusyIndicator and UserControl

HiI am doing a SilverLight (4) Application. In that, I want to create a UserControl which contains a BusyIndicator. In my MainPageView.xaml, I ve included this UserControl. In MainPageViewModel constructor , i am making isBusy property to true. On button click, i want to set IsBusy to false.How can i do this using MVVM. Please help if anybody knows the answer